X Digital Subscriber Line (xDSL) is a technology for high-speed data transmission over a telephone twisted-pair cable, or unshielded twisted pair (UTP). For a passband transmission xDSL, a frequency division multiplexing technology is used to enable the xDSL to coexist with a plain old telephone service (POTS) on a same twisted pair. The xDSL occupies a high frequency band and the POTS occupies a baseband part below 4 kilohertz (KHz). At a signal receiving end, a splitter is used to separate a POTS signal from an xDSL signal. A system providing access for multiple xDSLs is referred to as a digital subscriber line (DSL) access multiplexer (DSLAM).
As a result of electromagnetic induction, multiple signals connected by a DSLAM cause interference to each other, which is referred to as crosstalk (Crosstalk). There are two types of crosstalk, including far-end crosstalk (FEXT) and near-end crosstalk (NEXT). FIG. 1 is a schematic diagram of far-end crosstalk and FIG. 2 is a schematic diagram of near-end crosstalk. In the figures, TX represents a signal transmitting end, RX represents a signal receiving end, CO represents a central office, CPE represents a user side, downstream represents a downstream signal, and upstream represents an upstream signal.
Energy of both near-end crosstalk and far-end crosstalk increases as a frequency band goes higher. In prior-art xDSL technologies, such as Asymmetric DSL (ADSL), ADSL2, ADSL2+, Very High Bit Rate DSL (VDSL), and VDSL2, frequency division duplex (FDD) is used for upstream and downstream channels, impact of NEXT on system performance may be ignored and there mainly exists impact of FEXT. As xDSL uses an increasingly wide frequency band, impact of FEXT on transmission performance of VDSL2 is increasingly severe. At present, a vectoring technology has been introduced in the industry to enable joint transmission and reception at a central office (CO) so as to cancel FEXT crosstalk.
A VDSL2 standard defines eight configuration profiles, which are 8a, 8b, 8c, 8d, 12a, 12b, 17a, and 30a. In a vector DSL system, there are also lines of multiple profiles. For example, when a 17a line (that is, a line whose profile is 17a) coexists with a 30a line (that is, a line whose profile is 17a), on one hand, there is still an out-of-band spectrum above 17 megahertz (MHz) for the 17a line, and although a low-pass filter may be used, power of a 17-21 MHz out-of-band spectrum is still above −80 decibel-milliwatts per hertz (dBm/Hz). In a downstream direction, the out-of-band spectrum for signal transmission over the 17a line causes relatively strong FEXT interference to frequencies above 17 MHz of the 30a line. On the other hand, the spectrum above 17 MHz of the 30a line also causes FEXT interference to the out-of-band spectrum of the 17a line. When a modem at a receive end of the 17a line uses 1× sampling, this part of out-of-band interference is aliased to the inband spectrum of the 17a line, resulting in a decrease of a rate of the 17a line.
The foregoing FEXT crosstalk between the 17a line and the 30a line affects effects and stability of fiber to the customer (FTTC) acceleration.